The present invention relates to a shrink packaging material. More particularly, the present invention relates to a plastic packaging film prepared from a defined ethylene-.alpha.-olefin copolymer as a main component, which is excellent in heat shrinkability at relatively low temperature and little varying in thickness.
Heretofore known are such heat shrinkable films as polyvinyl chloride film, biaxially stretched polypropylene film, polyethylene film and so on. Among these films, polyethylene film is manufactured by the so-called inflation method in which a tubular film is extruded from a circular die and directly blown-up under gas pressure. This film, because of low production cost, high strength at the heat sealing joint and other advantages, has been widely used in shrink-wrapping applications.
The heat shrinkable polyethylene film produced by the inflation method, because of the very nature of the film-forming technique involved, has not been subjected to effective molecular orientation so that the film is inadequate in strength, high in elongation and shows a high rate of shrinkage only at high temperatures close to its melting point, thus being not fully satisfactory in such various properties as required for shrink packaging film.
The so-called irradiated polyethylene film which has been offered as an improvement over the above film in regard to the disadvantages just mentioned is a film which is obtainable by irradiating a polyethylene film with ionizing radiation so as to induce intermolecular crosslinking and, then, stretching the same. This irradiated polyethylene film has effects due to molecular orientation caused by stretching, showing increased strength and decreased elongation, for instance, but is disadvantageous in that it has a poor heat sealability, does not permit the reclamation and reuse of scraps, and is costly because of the radiation treatment required.
For the purpose of overcoming these disadvantages, the present inventors conducted a detailed study of the heat shrinkable polyethylene film from technical as well as material points of view and previously invented a heat shrinkable polyethylene film with improved heat shrinkability by orientation while retaining the desirable properties of polyethylene film such as good heat sealability and high impact resistance and a method for production of such heat shrinkable polyethylene film (U.S. Pat. No. 4,354,997 and No. 4,463,153).
Unlike the so-called inflation-molded film, the film obtainable in accordance with this prior invention develops the shrinkage and shrinkage stress necessary for shrink packaging even at a temperature below its melting point to establish an intimate contact with the object being packaged but the technique is not necessarily satisfactory in tube stability during stretching operation and in the uniformity of stretching result with a large variation in thickness. Moreover, the orientation effect induced by stretching is not fully realized so that the low-temperature heat shrinkability of the film is still inadequate. Therefore, when the object to be packaged is one, such as raw meat, that is liable to undergo degradation at a relatively low temperature region of about 85.degree. to 90.degree. C., intimate shrink packaging results cannot be attained with such film.
It is an object of the invention to provide a heat shrinkable polyethylene film which is little varying in thickness and has a good heat shrinkability at low temperature.
This and other objects of the invention will become apparent from the description hereinafter.